add 4sq supported optional param for users_venuestats to take in a friend id

Hi Matt,

Updated my users_venuestats endpoint. The gem's method interface `@client.venuestats()` hasn't changed since "self" is now the default parameter. As Foursquare's API supports passing in a friend's user_id, my commit updates the method and test coverage to support passing in a friend's user_id.

Let me know if you have any questions.

Thanks,
-Rex